U.S. Pat. No. 5,138,971 discloses a web charging apparatus, i.e. an apparatus that deposit electrostatic charges (ESC) on a moving web, preceding a coating apparatus so as to improve the affinity and adhesion of a coating solution on the web by depositing unipolar electrostatic charges on the surface of the web before the coating solution is applied on the surface. In the web charging apparatus, corona discharge is established with a DC power supply between wire electrodes and the grounded roller, while the wire electrodes are extended perpendicular to the direction of the moving web. The web is supported on a grounded roller or drum shaped electrode, functioning as a grounded electrode relative to the wire electrodes, so that the unipolar electrostatic charges can be deposited on the web. The apparatus according to this patent has the advantage that the coating solution can be easily applied on the web at the start of the coating and it is also possible to prevent the coating solution from being applied too thick. A corona treatment, however, offers several difficulties. One of the important difficulties thereof, associated with the manufacture of photosensitive materials is the occurrence of non-uniformity in photographic coating layer resulting from an uneven distribution of the electrical charge. This originates from non-uniform discharges. Since the photographic layer is a relatively thin layer of a few dozens of microns in thickness, the non-uniform coating severely affects the photographic characteristics as the density unevenness is recognised at the developed sample, such that the photographic products are markedly deteriorated in quality ("blur").
The uneven distribution of the electrical charge can be improved by including a second grounded back-up electrode behind the wire electrodes in the web charging apparatus. It is also known to use a so-called plasma glow discharge process at atmospheric pressure conditions to improve the adhesion of various layers on a substrate. Examples of the use of this process are the treatment of photographic base papers or polymer film supports prior to applying the various layers containing the photographic sensitive emulsions. For example EP-A 0 467 639, U.S. Pat. No. 5,403,453 and EP-A 0 821 273 are directed to this type of process. All these processes have in common that it applies an AC power supply while a noble gas is present between the web and the electrodes. Only the U.S. Pat. No. 5,403,453 also mentions the use of atmospheric air.
The stability of the atmospheric pressure glow discharge with parallel electrode plates (see EP-A 0 467 639 and U.S. Pat. No. 5,403,453) can be realised with expensive gas atmospheres containing mainly helium gas. In case the gas impurities become too high or the power source is outside a specific frequency range the glow discharge may become unstable and a less preferred discharge is obtained like the filamentary discharge. Accordingly, the prior art processes are only functioning within relatively narrow sets of conditions.
EP-A 0 821 273 discloses a method to obtain a plasma glow discharge at atmospheric pressure conditions (mainly in helium gas) between a series of thick Corona electrodes and a grounded drum shaped electrode. The adhesion improvement of the lowest emulsion layer at a polymer film support which is treated with plasma was shown at relatively low linespeeds of 10 m/min.